Insulating panels are known and have been described in, for instance, U.S. Pat. No. 3,449,879 as invented by BLOOM, wherein one longitudinal side defines a groove or slot and the other side a tongue so that a wall can be made by adjoining the tongue of one panel to the groove or slot of an adjacent panel. This system of grooved and tongued sides to build in mortices is extensively used. However, Applicant has found that in such systems made from foam products, such as insulating panels of expanded polystyrene and the like, the panels tend, for one reason or another, to slide with respect to each other and form uneven ends that are difficult to match with top sills.